There is known a plating apparatus configured to hold a substrate, such as a wafer, with a substrate holder and to immerse the substrate in a plating solution retained in a plating bath. As shown in FIG. 14, the substrate holder includes a plurality of inner contacts 100 that are brought into contact with a peripheral portion of a substrate W, and a plurality of outer contacts 101 coupled to the inner contacts 100, respectively. Wires 104, which provide electric connection between the inner contacts 100 and the outer contacts 101, are located in the substrate holder. When the substrate holder is disposed at a predetermined position in the plating bath, the outer contacts 101 are brought into contact with a power feeding terminal 103 that is coupled to a power supply 102. Electric current flows through the outer contacts 101 and the inner contacts 100 to the substrate W, so that a metal film is formed on a surface of the substrate W in the presence of the plating solution.
If an electric resistance between a certain inner contact 100 and the substrate W (which will be simply referred to as an electric resistance of inner contact 100) is extremely high or extremely low, uneven current flows through the inner contacts 100 to the substrate W, possibly causing a problem of non-uniform film thickness in a substrate surface.
Thus, in order to check whether or not the inner contacts 100 have abnormal electric resistances, a combined resistance including electric resistances of two inner contacts 100 is measured. When the substrate holder is holding the substrate W, the inner contacts 100 are electrically connected to each other through a conductive layer (e.g., a seed layer) of the substrate W. Specifically, when a resistance-measuring device is coupled to two of the outer contacts 101, an electric circuit is formed to electrically connect the substrate W, two inner contacts 100, and the resistance-measuring device. Therefore, the resistance-measuring device can measure a combined resistance including the electric resistances of the two inner contacts 100. If the combined resistance is not within a predetermined allowable range, it is judged that the inner contacts 100 have abnormal electric resistances.
The resistance-measuring device can measure the combined resistance including the electric resistances of the two inner contacts 100, but cannot measure each one of the electric resistances of the two inner contacts 100. Therefore, it is not possible to determine whether one of the two inner contacts 100 is in an abnormal contact state or both of the two inner contacts 100 are in an abnormal contact state. Moreover, even if one of the electric resistances of the two inner contacts 100 is extremely low and the other is extremely high, it is impossible to find out a problematic contact state of the inner contacts 100 as long as the combined resistance falls within the predetermined allowable range.
If a certain inner contact 100 has an extremely high electric resistance or an extremely low electric resistance, non-uniform electric current is passed to the substrate W during plating of the substrate W, thus preventing a uniformity of a thickness of a metal film formed on the surface of the substrate W.